ASU Remote Sensing

An impressive complement to the point and 1-dimensional profiling instrumentation is the lidar (manufacturer: Coherent Technologies) acquired in 2002 through a (DURIP) grant from the Department of Defense (PIs: R. Calhoun and H.J.S. Fernando). In this instrument, pulses of laser light of a given frequency are sent out into the atmosphere and returns are collected. Ambient aerosols scatter the laser light and a fraction of the light returns to the receiver/emitter. After a single pulse is emitted, the lidar waits for returns for a time corresponding to the outer length of its range. Modern coherent Doppler lidars have high pulse repetition frequencies (PRF). For example, Arizona State University’s (ASU) lidar emits 500 pulses per second with strength 2mJ. Because the returns are collected in discrete periods or “bins,” a distance can be associated with each time “bin” according to the distance that light can travel in the time measured. Therefore, the data is broken into range-gates. For example, a coherent Doppler lidar can have 100 range gates, each 65m long, for a total range of 6.5 km. Owing to its eye-safe infrared laser beam (2 m wave length; 2mJ/pulse at 400-500Hz) and direct measurement of aerosol backscatter, ASU lidar is ideally suited to measure radial winds and aerosols in urban environments. Three-dimensional, roughly hemispherical regions of the atmosphere can be scanned within minutes. ASU is one of the first universities to acquire a commercial (turn-key) lidar of this type for research purposes.

The velocity resolution of lidar is ~ 0.5 ms-1 and the spatial resolution (gate size) is ~ 50m in radial and ~ 1 m in azimuth. The range is ~ 10-15 km, with a maximum scan speed of 20o per second. The measurements can be made as close as one meter to the ground, if there are no obstacles.